Aw man, I told ya not buy one from China... oh, wait a minute.. I did...
This is what I would do... look for any bolts on the gimbal, your laptop, anything etc that look to be the same size... temporarily remove them and test fit them to see if they are the right fit for the larger hole(s)... if you get one that fits perfect it'll speed up figuring out what size it is.
Where I am I have a local store Specialty Tool and Bolt and they have all sizes and lengths of tiny bolts.. stainless, aluminum whatever... that would be my first stop to get a replacement. Do you have anything like that available in your area?
Another possibility... Is there a way to remove the new motor and install it your original aluminum gimbal bracket. If it's not too difficult tha might be a good option although I don't know the realty of it and we don't want to make things worse.
If that's not an option I would probably buy this DJI bolt kit from amazon... I imagine it has to have the right size bolts
https://www.amazon.com/DJI-Phantom-Part-41-Screw/dp/B00ZW0NGAE
In the mean time I would probably wrap a small piece of teflon tape or aluminum foil around the bolt to thicken it up and screw it in to temporarily hold the cover in place
Here's another thread asking bolt sizes, it has some bolt sizes mentioned but I don't know..
Standard - What screw size?